Absorbance, Reflectance and Transmission

Absorbance is a measure of the amount of light absorbed by a sample as a beam of light passes through it. In this process, a chemical species is excited from a lower to a higher energy level, as such:

A chemical species absorbing a photon and undergoing a spectroscopic transition from a lower to an upper energy level.

This is what a UV-Vis absorbance spectrum looks like:

The size of the peaks in an absorbance spectrum is proportional to concentration, thus absorbance spectra can be used for quantitative analysis. Transmittance is the amount of light transmitted by a sample and is mathematically related to absorbance. Reflectance measures the amount of light that reflects from the surface of a sample. These measurements can be used to determine the chemical structure and properties of surfaces and surface-adsorbed species.

Absorbance, transmittance, and reflectance measurements are typically made with monochromators (i.e., spectrographs that have an exit slit and a single-point detector).

When choosing a single-point detector, you must first consider the wavelength region to be studied. Photomultiplier tubes (PMTs) are used in the ultraviolet, silicon detectors in the visible, indium gallium arsenide (InGaAs) and lead sulfide detectors in the near-infrared region, and mercury cadmium telluride (MCT) in the mid-infrared region. Princeton Instruments sells single-point detectors made of each of these materials.

Absorbance, transmittance, and reflectance are all calculated by taking measurements of the light beam intensity before and after the sample. In a double-beam spectrometer, both measurements can be made at the same time by chopping the beam and alternately sending the beam to and around the sample. If the two measurements are made at separate points in time, with the sample present and absent from the light beam, it is called a single-beam setup. Single-beam spectra can suffer from artifacts and photometric errors because of the time lag between the two measurements. This is why a dual-beam configuration is recommended whenever possible.

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A generic absorbance/transmittance or reflectance measurement system consists of: